Digital transversal filters of the "tapped delay line" kind are used to form linear combinations of a given number of incoming digital values. Such filters are only able to achieve effective echoelimination and/or equalization in telecommunication systems when the signal paths in which the filters are located contain no non-linear devices. An echoelimination filter is normally coupled between a transmitting branch and a receiving branch which are connected to a 2-to 4-wire junction. An equalization filter is normally coupled in the receiver branch. The transmission branch includes a D/A converter, whereas the receiver branch includes an A/D-converter. It has been found that the D/A and A/D converters are the major sources of non-linearity in conjunction with echo-elimination and equalization.
According to one known method, see for instance EP, Al, 0.240.055, non-linearities are compensated with the aid of a separate correction device connected downstream of an A/D-converter in a receiver branch. This device generates correction values which are added to the converter output values. Another possibility of avoiding problems caused by non-linear converters is the use of table filters instead of transversal filters. Table filters eliminate echo and achieve equalization despite non-linearities. Both methods, however, require relatively complicated and bulky equipment.
An A/D-converter which operates with successive approximations normally includes a D/A-converter having a plurality of binary-weighted devices, for instance capacitors or current sources. Non-linearities occur in converters of this kind because the binary-weighted devices are, in practice, not binary-weighted to sufficient accuracy. One method of improving linearity is to adjust the values of the weighted devices more accurately during manufacture, e.g. by cutting away parts of the capacitors using a laser. This method, however, is expensive. Furthermore, the component values change with time in depending on temperature, etc., and consequently no durable linearity can be achieved.
An A/D-converter which operates with successive approximations can also be self-calibrating, meaning that the values of the binary-weighted devices are monitored and corrected at regular intervals , e.g. after each conversion cycle. Correction, for instance, of the capacitance values is effected by activating or deactivating small parallel-connected capacitors in order to respectively increase and decrease the capacitance. This also requires the provision of additional logic devices for carrying out requisite calibration cycles and for producing correction signals. At the same time, the conversion capacity is reduced resulting from carrying out the calibration cycles. Selfcalibration is known from, for instance, the article "Self-calibration and oversampling make room for more digital circuitry on monolithic ADCs", EDN, Oct. 15, 1987.